Prostate-Specific Antigen (PSA) is a protein produced by both normal and cancerous cells within the prostate gland. A blood test measures this protein level, making it the primary tumor marker used to monitor prostate cancer activity. After curative treatment like radiation therapy, tracking the PSA level assesses the treatment’s effectiveness and long-term success. Unlike surgical removal, radiation (External Beam Radiation Therapy or Brachytherapy) leaves the prostate gland in place. Because remaining healthy tissue continues to produce PSA, the level will not drop to zero, but instead follows a unique, slower trajectory compared to post-surgical results.
The Timeline of PSA Decline After Radiation
The decline in PSA following radiation is a gradual process that unfolds over many months and sometimes years. This slow decrease occurs because radiation causes prostate cells to die off over time, rather than instantly eliminating them. The remaining benign prostate tissue continues to produce some PSA. The PSA level may not reach its final lowest point until 18 months to three years after treatment completion.
During this period, some patients experience a “PSA bounce,” which is a temporary, non-cancer-related rise in the PSA level. This bounce typically occurs within the first two years following treatment, often around 16 months, and is believed to be related to inflammation caused by the radiation.
A PSA bounce is usually a temporary event, with the level subsequently returning to its original low point or continuing its decline. This temporary rise is distinct from a true recurrence, and studies suggest that experiencing a bounce may be associated with improved long-term outcomes. Clinicians advise against intervening or making major treatment decisions based on a single, isolated PSA rise that fits the profile of a bounce.
Defining Treatment Success The PSA Nadir
The most meaningful metric for success after radiation therapy is the PSA Nadir, defined as the lowest PSA value achieved after treatment completion. Since the prostate gland remains, a successful outcome does not mean the PSA level will become undetectable, unlike after a radical prostatectomy. The goal is to achieve a very low and stable nadir that indicates the cancer cells have been effectively eliminated.
For patients who did not receive concurrent hormone therapy, a successful post-radiation nadir is generally considered less than 1.0 ng/mL. A lower nadir is a stronger indicator of a successful outcome and better long-term prognosis. Achieving a nadir of 0.5 ng/mL or less is often associated with the highest rates of long-term disease-free survival.
The speed and depth of the nadir carry prognostic information. Patients whose PSA continues to decline to these low levels over time have had a favorable response. Conversely, a nadir persistently above 1.0 ng/mL may signal a higher risk of recurrence. The nadir value serves as the baseline against which all future PSA readings are compared.
Recognizing Biochemical Recurrence
A rising PSA after the nadir is established indicates treatment failure, known as biochemical recurrence. To create a standardized definition, the Radiation Therapy Oncology Group (RTOG) and the American Society for Therapeutic Radiology and Oncology (ASTRO) developed the Phoenix Consensus Definition. This definition is the current standard for diagnosing recurrence following radiation therapy.
Biochemical recurrence is defined as the PSA level rising by 2.0 ng/mL or more above the established nadir. For example, if the nadir was 0.4 ng/mL, recurrence is diagnosed when the PSA reaches 2.4 ng/mL (0.4 + 2.0). This threshold provides a reliable indicator that the cancer has returned, differentiating true treatment failure from temporary fluctuations or a PSA bounce.
The diagnosis of recurrence is made “at call,” based on the single reading that meets this threshold, and the date of failure is the date of that test. If this threshold is met, the speed at which the PSA is rising, known as the PSA doubling time, becomes a significant factor. A faster PSA doubling time is associated with a more aggressive recurrence and influences decisions about subsequent imaging and treatment options.
Ongoing Monitoring and Follow-Up Testing
Consistent surveillance is necessary after radiation to ensure any potential recurrence is identified early. PSA testing is typically performed on a standardized schedule to track the decline toward the nadir and monitor for any subsequent rise. Guidelines usually recommend testing the PSA level every three to six months for the first few years after treatment.
After the initial phase, testing frequency often shifts to an annual basis for long-term monitoring. The purpose of this surveillance is to detect a sustained rise that meets the Phoenix definition of biochemical recurrence. If the PSA level rises significantly and meets the threshold, the next step is often to confirm the result with a second test.
If recurrence is confirmed, advanced imaging techniques, such as a Prostate-Specific Membrane Antigen (PSMA) PET scan, may be used to locate where the cancer has returned. Identifying the location of the recurrence, whether local within the prostate area or spread elsewhere, is crucial for guiding further treatment decisions.